As municipal land waste areas continue to become completely filled, alternate methods of refuse disposal assume an increasingly large importance. The aggrandizement of this problem, moreover, results in efforts to totally destroy the refuse, especially through burning. This undertaking, however, must comply with current environmental restrictions. Yet, burning the material and thus attempting to recover the heat produced as especially tantalizing goal in this age of excessively high energy costs.
The environmentally acceptable burning of refuse and other wastes constitutes the objective of many drastically different types of incinerators. Almost all aspects of the combustion process and equipment have engendered widely divergent techniques and components in attempting to control the burning and, more importantly, the resulting air pollutants.
To begin with, various incinerators impose specific requirements upon the refuse which they will burn. Some incinerators require the removal of various noncombustible components prior to the entry of the remaining portions into the combustion chamber. The sorting process, of course, requires the expenditure of substantial economic resources for the labor or machines that accomplish the task. It also slows down the overall disposal system.
Other incinerator systems actually require the shredding of the waste before it can burn. The grinding, of course, entails the use of expensive machinery to reduce the collected waste into an acceptable form. Furthermore, prior to the commencement of the grinding, a selection process must remove at least some egregious components; gasoline cans, for example, can explode and destroy the grinder and, perhaps, people in the near vicinity. Accordingly, the additional grinding and, usually, sorting steps impose additional machinery, costs, and time onto the disposal process.
Reducing the waste into a shredded form apparently has the objective of creating a uniform type of material which will burn predictably. This permits the incinerator designer to construct the apparatus with the knowledge that it will have a specific known task to accomplish. However, once in the incinerator, the shredded waste creates an additional problem; it permits the very rapid burning of the material at possibly excessive temperatures. The resultant high gas velocities within the chamber can entrain particulate matter into the exhaust stream. These large amounts of particulates will then escape the incinerator to create prohibited, or at least undesired, smoke.
The main combustion chambers that the entering refuse initially encounter have also witnessed a wide degree in variation of designs. Some incinerators place the refuse upon a grate bed. This allows the air or other oxygen-containing gas to readily and uniformly intermingle with the refuse to assure complete combustion. However, unburned ash, plastics, wet refuse, and liquids may simply drop down through the grates to the bottom of the incinerator. There they undergo combustion and can provide excessive heat to the incinerator's lower surface and grating structure, possibly damaging them. They can also stay and otherwise alter the actual floor of the chamber.
A hearth, or refractory, floor represents an alternative to the grate support for refuse. However, a hearth floor interposes other problems in attempting the effective and efficient combustion of refuse.
Initially, the refuse upon the floor must receive an even distribution of oxygen in order for the bulk of the material to burn. This throughput of oxygen does not occur if the air simply passes into the combustion chamber over the burning refuse; it must enter underneath the waste material and disperse throughout. The uniform dispersion of the air into the waste requires the placement of air nozzles within the hearth floor itself. However, the heavy refuse sitting upon the floor has shown an unmistakeable propensity to clog and destroy the effectiveness of the air-introducing nozzles. As a result, the refuse does not undergo efficient and thorough combustion.
To prevent the clogging of nozzles in a hearth floor, some incinerators force the air through at a high velocity. This hopefully avoids the clogging problem. However, the fast-moving gases again display a propensity to entrain particles and produce smoke. Furthermore, the high velocities have a tendency to create a "blow torch" effect and produce slag. The slag may then stick to the hearth floor and interfere with the chamber's subsequent operation.
Further, incinerators currently in use also employ drastically different geometric designs for the initial combustion chamber. For example, some use a tall compartment occupying a relatively small horizontal area. Others utilize cylindrical chambers with the main axis of cylindrical symmetry lying horizontally. Most also use chambers with a minimal volume to permit the burning of the intended refuse. All of these factors, again, however, increase the velocity of gases passing through and thus the entrainment of particulate, smoke-producing material.
Many incinerators also attempt to control the amount of air entering the first combustion chamber. They select the quantity of oxygen and thus, presumably, the combustion rate within the main chamber. Thus, some incinerators use an amount of air far in excess of the quantity required to stoichiometrically burn the refuse inside. Others employ a "starved air" process and permit the entry of substantially less air than dictated by stoichiometry.
The large amounts of air in the former system again help to entrain particulate matter. These excess air systems attempt to control this problem by choking the output of the main combustion chamber. However, a small throat itself increases the gas velocity in the vicinity which can thus defeat the main goal of avoiding the entrainment of particles.
The starved air systems, in comparison, do not provide sufficient oxygen to achieve the combustion of the material placed inside. However, the heat developed in the main chamber effects the volatilization of much of the introduced hydrocarbon material. As these hydrocarbons assume the vapor form, they can create very substantial positive pressure within the main combustion chamber. These pressures, as the gases inside attempt to escape, actually create high velocities. These velocities again entrain particulate matter which results in smoke.
Furthermore, the positive pressures inside the starved-air combustion may also force its internal gases into the area immediately surrounding the chamber. In an enclosed room, the combustion gases pass into areas occupied by the operating personnel. Moreover, the lack of oxygen in the starved-air process does not permit the burning hydrocarbons to convert to water and carbon dioxide; carbon monoxide frequently represents a very substantial component in this type of chamber. The internal positive pressures can then force the carbon monoxide into the area where the operating personnel may breathe it. Accordingly, the starved-air system should should typically have a location outside of a building or in an extremely well ventilated area.
The incinerators of the days before environmental concern simply released their exhaust gases from the combustion chamber into the atmosphere. The obviously detrimental effect of these gases upon the environment has resulted in prohibitions on their continued use. Moreover, it has led to the development of additional techniques for controlling the pollutants produced in the combustion chamber.
Efforts to control pollution have often centered upon the use of a reburn tunnel to effectuate further combustion of the main combustion chamber's exhaust. The gases, upon departing the main combustion chamber, immediately enter the reburn unit. The tunnel may include a burner to produce heat and a source of oxygen, usually air, to complete the combustion process. The additional oxygen, of course, represents an essential ingredient for the starved-air incinerators. Depending upon the material introduced in the main chamber, the reburn unit provides a set amount of fuel to the burner and a specified amount of oxygen.
Typically, the incinerator's manufacturer sets the burner level and the amount of oxygen for the amount and kind of waste he expects the incinerator to receive. When the main chamber does, in fact, receive the expected refuse, the reburn unit can effectively provide a "clean" exhaust.
However, deviations in the amount or quality of the refuse place unexpected strains and requirements on the reburn unit. This can cause the unit to lose its ability to prevent atmospheric pollution. When this occurs, the incinerator system, with the reburn unit, will release unacceptable amounts of pollutants into the atmosphere.
Furthermore, many incinerators, while attempting to avoid degrading the environment, have also sought to recover the heat produced by the combustion. Some try to capture heat directly within the main combustion chamber. Others choose to locate a boiler past the reburn unit, where employed. Maximizing the recovery of the produced energy while avoiding substantial pollution, however, has not yet yielded to a satisfactory solution.